Cleaned up the /chapters directory

Addresses #36 

I removed the old `maptime.geojson` file since we're using `/_data/chapters.json` now so we don't maintain two separate files.  Moved the `README` into the `/chapters` and fixed a few broken links to simplify onboarding.  I also moved `maptime-marker.png` into `/img` so it's easy to find.

cc @geografa